不懂回文自动机的可以看这篇博客。
用P5496 【模板】回文自动机(PAM) 的方法(这篇题解)求出以每个位置结束的回文串个数,答案为所有位置的和。
#include <cstdio>
#include <cstring>
const int MAXN = 500000 , MAXK = 26;
char str[ MAXN + 5 ];
struct Palindrome_Automaton{
int Size , Last , Root0 , Root1 , Trans[ MAXN + 5 ][ MAXK + 5 ] , Link[ MAXN + 5 ];
int Len[ MAXN + 5 ] , Cnt[ MAXN + 5 ];
Palindrome_Automaton( ) {
Root0 = Size ++ , Root1 = Size ++; Last = Root1;
Len[ Root0 ] = 0 , Link[ Root0 ] = Root1;
Len[ Root1 ] = -1 , Link[ Root1 ] = Root1;
}
void Extend( int ch , int dex ) {
int u = Last;
for( ; str[ dex - Len[ u ] - 1 ] != str[ dex ] ; u = Link[ u ] );
if( !Trans[ u ][ ch ] ) {
int Newnode = ++ Size , v = Link[ u ];
Len[ Newnode ] = Len[ u ] + 2;
for( ; str[ dex - Len[ v ] - 1 ] != str[ dex ] ; v = Link[ v ] );
Link[ Newnode ] = Trans[ v ][ ch ] , Trans[ u ][ ch ] = Newnode;
Cnt[ Newnode ] = Cnt[ Link[ Newnode ] ] + 1;
}
Last = Trans[ u ][ ch ];
}
void Build( char *str ) {
int len = strlen( str );
for( int i = 0 ; i < len ; i ++ )
Extend( str[ i ] - 'a' + 1 , i );
}
int Calc( ) {
int Ans = 0;
for( int i = 1 ; i <= Size ; i ++ )
Ans += Cnt[ i ];
return Ans;
}
}PAM;
int main( ) {
scanf("%s", str );
PAM.Build( str );
printf("%d", PAM.Calc( ) );
return 0;
}